What everyone else said.
I'm 45 and although I did have horses in my youth, I'm not sure that helped any! I was one of these kids who was so horse crazy, my parents just gave in and bought me a horse that lived in our backyard. I had maybe a year of lessons every other week. I didn't have a clue what I was doing and learned the hard way. I had to sell my horse when I went off to university over 25 years ago. I studied for a LONG time, worked on building a career in a competitive field, had kids... a lot of time went by. My daughter started riding 6 years ago and last fall, I "bought her" a horse (well, a little for me too!) and started taking lessons again just a few weeks ago. I might as well be a total beginner.
When I had my horse as a teenager, I rode western and now I'm learning English. In the last 25 years, horsemanship has changed so dramatically that basically, nothing I knew then (and I knew very little) applies today. Except maybe grooming which hasn't changed much, but that's about it. Riding is different (esp. since I switched disciplines), feeding is different, turnout is different, hoof care is different, stabling is different. I honestly feel I had to approach the whole thing as a total beginner. But I'm loving every minute of it!
As an adult, you will learn faster and you're still young enough that your body is supple and fit (unlike us 40somethings!). I see too many kids starting very young, but not really showing a lot of improvement because they lack the fine motor skills and/or the motivation to really focus on horses. My daughter is one of those kids. She is involved in so many other things and doesn't want to just do horsey stuff. Which is fine - she should explore her interests at that age, but it means she is a bit scattered.
You will do fine, especially with two rides per week. Depending on your level of fitness, expect some soreness at first, just because you'll be working new muscles, but that should go away soon. Let us know how you're doing!